This invention relates to high frequency electrical power transmission and especially to a reflectionless termination or dummy load for coaxial transmission lines, commonly called TEM lines. More particularly the invention relates to an air cooled type termination.
Frequently, in testing transmitters or in measuring RF power, it is necessary to terminate a TEM line in a reflectionless termination or dummy load. The termination must be capable of absorbing and dissipating the RF power being transmitted, in the form of heat. The problem of providing a reflectionless termination is very complex when the termination must dissipate power in the order of kilowatts. For example, a coaxial line or TEM line has predetermined physical dimensions that determine the electrical characteristics that must be matched by the termination in order to prevent undesirable reflection of radio frequency waves.
Among the types of TEM line terminations currently employed, many use a tapered conductor, or horn that is connected to the outer conductor of the coaxial line and that tapers logarithmically. One end of the horn contacts a cylindrical resistor that is connected to the inner conductor of the coaxial line. Due to the logarithmic taper and because the horn is connected to the resistor, this combination restricts the high power frequency response that may be obtained.
Since the size of the inner and outer conductors in a coaxial configuration determines the cutoff frequency as well as the impedance of the resulting TEM main mode of propagation, the overall diameter must remain as small as possible to extend the usable high frequency response of the device.
In terminations that utilize air cooling it is necessary to have a substantial flow rate for the air passing around the heated elements in order to achieve satisfactory heat dissipation. In such prior art devices, because the resistor is connected to the inner conductor of the transmission line, a limited surface area is provided for contact with the heat transfer medium.
The device of the present invention, however, provides a substantial improvement over these prior art devices, particularly as to its ability to absorb relatively large amounts of electromagnetic power, to dissipate the resulting heat in an efficient manner, and to maintain a high frequency response.